1. Field of the Invention
The present invention relates to a refrigeration cycle and, more particularly, to a control of a refrigeration cycle in which a non-azeotrope refrigerant is used as a working fluid.
2. Description of the Related Art
First, the problem which arises when a non-azeotrope refrigerant is used as a working fluid will be explained. The non-azeotrope refrigerant is a refrigerant in which two or more types of refrigerants having different boiling points are mixed, and has characteristics shown in FIG. 1. FIG. 1 is a vapor-liquid equilibrium diagram illustrating characteristics of a non-azeotrope refrigerant in which two types of refrigerants are mixed. The horizontal axis indicates the composition ratio of a refrigerant having a low boiling point, and the vertical axis indicates temperature. In the diagram pressure is used as a parameter. The composition ratio X=0 indicates that only a high-boiling-point refrigerant exists, and the composition ratio X=1.0 indicates that only a low-boiling-point refrigerant exists. In a mixture refrigerant, as shown in FIG. 1, a saturation liquid line and a saturation vapor line are determined by the composition thereof. The area below the saturation liquid line indicates the supercooled state, and the area above the saturation vapor line indicates the superheated state. The portion surrounded by the saturation liquid line and the saturation vapor line is a two-phase state of liquid and vapor. In FIG. 1, X0 denotes the composition of a refrigerant charged in a refrigeration cycle. Points 1 to 4 indicate the typical points of the refrigeration cycle, and point 1 indicates a compressor outlet portion; point 2 indicates a condenser outlet portion; point 3 indicates an evaporator inlet portion; and point 4 indicates a compressor inlet portion.
An explanation will be given below of problems relating to leakage out of the refrigeration cycle, to variations in the composition of a refrigerant circulating in the refrigeration cycle in a non-steady state such as at the start-up time of the refrigeration cycle, and to operation control of a refrigeration cycle.
The leakage of a refrigerant out of the refrigeration cycle is not none even in a hermetically sealed type air-conditioner or refrigerator. In FIG. 1, point A indicates the two-phase portion in the refrigeration cycle, in which the liquid of composition Xa1 and the vapor of composition Xa2 exist. In the case that the refrigerant leaks out of a heat-transfer tube of a heat exchanger or from a connection tube of a component, the leaked refrigerant would be a refrigerant of composition Xa1 in the case of liquid leakage, and a refrigerant of composition Xa2 in the case of vapor leakage. Therefore, the composition of the refrigerant remaining within the refrigeration cycle differs depending upon whether liquid or vapor leaks.
FIG. 2 is an illustration of a problem caused by the leakage of a refrigerant to the outside. If liquid leaks, the remaining mixture refrigerant enters the state of X1 in which the ratio of a low boiling-point refrigerant is large; if vapor leaks, the remaining mixture refrigerant enters the state of X2 in which the ratio of a high boiling-point refrigerant is large. In FIG. 2, X0 indicates the composition of a refrigerant which is sealed in initially. Comparing a state having the composition ratio of X0 with a state having the composition ratio of X1 under the same pressure, the temperature in the state having the composition ratio of X1 is lower. Comparing a state having the composition ratio of X0 with a state having the composition ratio of X2 under the same pressure, the temperature in the state having the composition ratio of X2 is higher.
FIG. 3 shows general characteristics of a refrigeration cycle with respect to the composition ratio of the low boiling-point refrigerant. When the low boiling-point refrigerant composition ratio X becomes larger than the designed composition X0, the discharge pressure and the intake pressure become higher, and therefore capacity improves. In contrast, when the low boiling-point refrigerant composition ratio X becomes smaller than the designed composition X0, the discharge pressure and the intake pressure become lower, therefore capacity deteriorates.
Next, the problem in a non-steady state such as at the start of the refrigeration cycle will be explained. FIG. 4 illustrates the construction of the refrigeration cycle. Referring to FIG. 4, reference numeral 1 denotes a compressor; reference numeral 2 denotes a four-way valve; reference numeral 3 denotes a heat-source side heat exchanger; reference numeral 4 denotes a refrigerant pressure reducing apparatus; reference numeral 5 denotes an accumulator; and reference numeral 6 denotes a use-side heat exchanger. A non-azeotrope refrigerant is charged in. In FIG. 4, the refrigerant circulates in the direction of the solid-line arrow during the cooling operation, and circulates in the direction of the dashed line arrow during the heating operation. The pressure when the refrigeration cycle shown in FIG. 4 is started, and changes in the compositions of the circulating refrigerant are shown in FIG. 5. When the refrigeration cycle is started, the low-pressure side pressure decreases. This pressure reduction causes the low boiling-point refrigerant to be gasified from the liquid refrigerant remaining in the accumulator or the like, and the circulating refrigerant reaches a state in which the composition ratio of the low boiling-point refrigerant is large. When the composition ratio of the low boiling-point refrigerant becomes large as described above, both the discharge and intake pressures become higher, and the discharge pressure may happen to exceed an upper-limit value.
If the refrigerant leaks out of the refrigeration cycle in which a non-azeotrope refrigerant is used as a working fluid, as described above, the composition of the refrigerant remaining within the refrigeration cycle changes from the initial composition, i.e., from the designed composition for the apparatus depending upon leaked portions. Even if there is no leakage to the outside, there is a possibility that the composition of the refrigerant circulating within the refrigeration cycle may vary in the non-steady state of the refrigeration cycle.
Changes in the composition of the refrigerant within the refrigeration cycle cause problems; for example, capacity is varied, or pressure or temperature becomes abnormal. Therefore, the refrigeration cycle must be controlled properly.
Technology described below is available for controlling the refrigeration cycle in which a non-azeotrope refrigerant is used as a working fluid.
Disclosed in Japanese Patent Unexamined Publication No. 1-256765 is technology for making always constant the superheatedness of a refrigerant at an evaporator outlet constituting the refrigeration cycle even if the composition of the refrigerant within the refrigeration cycle varies due to leakage. More specifically, according to the technology proposed, the composition of the refrigerant circulating within the refrigeration cycle is determined by comparing the measured values of the pressure and temperature in a high-pressure liquid portion of the refrigeration cycle with the prestored temperature and pressure characteristics of a non-azeotrope refrigerant. Even in the above determined composition, the superheated degree is always maintained at the superheated degree before the composition is varied.
In another technology disclosed in Japanese Patent Unexamined Publication No. 1-200153, a compressor constituting the refrigeration cycle is a compressor of a variable rotation speed type, a pressure detection mechanism being disposed in the compressor discharge section so that the rotation speed of the compressor is controlled such that the pressure in the discharge section does not exceed a fixed value.
A conventional method of controlling a refrigeration cycle in which a single refrigerant is used is disclosed in Japanese Utility Model Unexamined Publication No. 47-27056, Japanese Patent Unexamined Publication No. 1-305272 and the like. These publications disclose a method of controlling the pressure to be constant.
As described above, in the refrigeration cycle in which a non-azeotrope refrigerant is charged, the composition of the refrigerant within the refrigeration cycle may vary when the refrigerant leaks out of the refrigeration cycle or during the non-steady operation of the refrigeration cycle. Therefore, the refrigeration cycle must be controlled properly in accordance with the composition of the refrigerant.
In connection with this, in the above-described related art, although the superheated degree of the refrigerant in the evaporator outlet of the refrigeration cycle is controlled to be constant even if the composition of the refrigerant is varied, no consideration has been given to the fact that the characteristics to be controlled are varied in accordance with the composition when the composition is varied. Further, although the discharge pressure is controlled so as not to exceed a certain value on the basis of the rotation speed of the compressor, no consideration has been given to the fact that the superheatedness of the refrigerant is controlled in accordance with the composition, for example, by changing the upper limit of the discharge pressure in accordance with the composition.
In the conventional method of controlling the refrigeration cycle in which a single refrigerant is used, as a matter of course, no consideration has been given to the composition of the refrigerant.